Conventional leather processing involves four important operations, viz., pre-tanning, tanning, post-tanning and finishing. It includes a combination of single and multi-step processes that employs, as well as expels, various biological, organic and inorganic materials as described by Germann (Science and Technology for Leather into the Next Millennium, Tata McGraw-Hill Publishing Company Ltd., New Delhi, p. 283, 1999). Conventional methods of leather processing typically involve 14–15 steps, comprising soaking, liming, reliming, deliming, bating, pickling, chrome tanning, basification, rechroming, basification, neutralization, retanning, dyeing, fat liquoring and fixing. Liming and reliming processes employ lime and sodium sulfide. These two processes purify the skin matrix by the removal of hair, flesh and other unwanted materials to produce pelt. The deliming process employs quaternary ammonium salts for neutralizing the alkalinity. The bating process purifies the skin matrix further by using pancreatic enzymes. The pickling process prepares the skin for subsequent tanning. The tanned skin matrix may be further retanned to gain substance, fat liquored to attain required softness and dyed to preferred shades. The conventional technique discharges enormous amount of pollutants, which accounts for nearly 98% of the total pollution generated from a tannery as analyzed by Aloy et al (Tannery and Pollution, Centre Technique Du Cuir, Lyon, France, 1976). The pollution includes biochemical oxygen demand (BOD), chemical oxygen demand (COD), total dissolved solids (TDS), sulfides, chlorides, sulfates, chromium, etc. This is primarily due to the fact that the conventional leather processing employs ‘do-undo’ process schemes, such as swell-deswell (liming-deliming); pickle-depickle (pickling-basification), rechroming-basification (acidification-basification) and neutralization-fixing (basification-acidification) as described by Bienkewicz (Physical Chemistry of Leather Making, Krieger Publishing, Malabar, Fla., 1983). In other words, conventional methods employed in leather processing subject the skin or hide to wide variations in pH. Such pH changes require the use of acids and alkalis, and lead to the generation of salts. This results in a net increase in COD, TDS, chlorides, sulfates and other minerals in tannery wastewaters as reported by Thanikaivelan et al (Journal of the Society of Leather Technologists and Chemists, 84, 276, 2000).
Conventional chrome tanning generally involves pickling, tanning using basic chromium sulfate (BCS), followed by basification processes. Spent pickle liquor has a high dissolved solid content and a considerable amount of chemical oxygen demand, since pickling involves the use of 8–10% sodium chloride salt along with sulfuric acid as reported by Aloy et al (Tannery and Pollution, Centre Technique Du Cuir, Lyon, France, 1976). The use of non-swelling acids in pickling has been reported by Herfeld and Schubert (Das Leder, 26, 117, 1975) in order to reduce total dissolved solids. Several better chrome management methods have been developed, for example, high exhaust chrome tanning as reported by Chandrasekaran (Leather Science, 34, 91, 1987); pickle-less tanning as given by Venba et al (Poster presented at 30th Leather Research and Industry Get-together, Chennai, 1995); chrome recovery and reuse as reported by Covington et al (Journal of the Society of Leather Technologists and Chemists, 67, 5, 1983); and closed pickle-tan loop system as described by Rao et al (Science and technology for leather into the next millennium, Proceedings of the XXV International Union for Leather Technologists and Chemists Societies congress, p. 295, 1999). The conventional method of post-tanning process involves 7–8 major steps comprising rechroming, basification, neutralization, washing, retanning, dyeing, fat liquoring and fixing. The post-tanning operation employs a pH range of 4.0–7.0 and a variety of chemicals. The post-tanning processes contribute to TDS, COD and heavy metal pollution as analyzed by Simoncini and Sammarco (Proceedings of the XXIII International Union for Leather Technologists and Chemists Societies congress, Germany, 1995). Post-tanning chemicals generally contribute to COD in large measure and this can be mitigated by the use of optimized quantities of high performing auxiliaries, which would lead to less discharge of bio-treatable residues. Selection of retanning and fat liquoring agents is based on their biodegradability and uptake behavior. However, these improvements are specific to a unit operation. Implementation of all the advanced technologies and eco-friendly chemicals involves financial input and machinery requirements as well. This calls for the development of integrated leather processing technology and reorganization of various steps in leather processing. Very few attempts have been made to reorganize the sequence of the leather processing steps. Thanikaivelan et al. have attempted to make leather in a narrow pH range from 4.0–8.0 (Journal of the Society of Leather Technologists and Chemists 84, 276, 2000; 85, 106, 2001). Thanikaivelan et al have developed a biochemically based three step tanning process in the pH range of 4.0–8.0 (Journal of the American Leather Chemists Association 98, 173, 2003). The three-step tanning process involves dehairing at pH 8.0 without employing lime, sodium hydroxide based fiber opening and pickle-less chrome tanning at pH 8.0. Saravanabhavan et al. have successfully developed a three step tanning method, which involves dehairing without employing lime and sodium sulfide, enzyme based fiber opening and chrome tanning at pH 8.0 (Green Chemistry 5, 2003, 707). However, no attempts have been made to swap the leather processing steps. The above-mentioned references disclose processes that are preliminary to prime tanning. Subsequently, the prior art processes require conventional post-tanning processing, involving 7–8 major steps comprising rechroming, basification, neutralization, washing, retanning, dyeing, fat liquoring and fixing.
The main objective of the present invention is to provide a novel transposed process for making leather.
Another object of the invention provides a novel transposed process, wherein the process does not require pickling and basification steps.
Yet another object of the present invention provides a transposed process wherein the post-tanning process does not require acid washing, rechroming, neutralization, washing and fixing.
Still another object of the present invention is directed to a transposed process that leads to significant reduction in chemical oxygen demand, total solids load and other pollution loads.
One more object of the present invention provides an inventive step in treating untanned pelts with syntans, fat liquors and dyes, which are conventionally considered as post-tanning wet processing chemicals, thereby providing an eco-friendly option for leather processing.